Devices whereby the local fluid rates in a fluidised bed are made to fluctuate

ABSTRACT

A method of operating a fluidised bed and apparatus for carrying out the operation by means of which the rate of flow of fluid entering the bed from particular regions of the grid is varied while the gross flow of fluid is maintained substantially invariant, to assist in avoiding the formation of preferred channels or paths through the bed by the entering gas or vapor.

White tates atent [1 1 Dong DEVICES WHEREBY THE LOCAL FLUID RATES IN AFLUlDISlED BED ARE MADE TO FLUCTUATE Inventor: Ian Dracup Doig,Roseville Chase,

New South Wales, Australia Assignee: Unisearch Limited, Kensington,

, Wales Filed: Apr. 23, 1973 App], No.: 353,888

Foreign Application Priority Data Apr. 27, 1972 Australia 8768/72 US.Cl. 55/77, 34/57 A Int. Cl BOld 53/06 Field of Search 55/77, 79, 74,179, 181,

[ Feb. 18, 1975 [56] References Cited UNITED STATES PATENTS 2,813,35111/1957 Gode] 34/57 A 3,503,184 3/1970 Knapp et a1. 55/71 3,605,2769/1971 Enters 34/57 A Primary ExaminerCharles N. Hart Allomey,Agent, orFirm-Browdy and Neimark 4 Claims, 3 Drawing Figures PATENTED E 3.8 67.114

sum 18F 2 FIGZ FIG] PATENTED FEB 81975 SHEET 20F 2 DEVICES WHEREBY THELOCAL FLUID RATES IN A ELUIDISED BED ARE MADE TOFLUCTUATE This inventionis an improvement in the method of operating fluidised bed processes inwhich a gas or vapor is delivered through a grid or other supportcontaining ports or passages for the gas or vapor to the base of a bedof discrete granular solid particles. (The term grid as used in thisspecification is to be taken to include any such support). In a typicalfluidised bed, the gas or vapor enters the bed of particles at a ratesufficiently high to raise the particles and impart to them individualor aggregate motions: a bed thus agitated by the gas or vapor is said tobe fluidised.

During the fluidisation the gas or vapor exchanges material or heatseparately or simultaneously with the solid particles in the bed besidesthe mechanical energy the gas or vapor delivers to the bed. In otherversions a liquid enters either with the gas or separately and transfersmaterial (often a solute) to the particles in the bed or serves to formdiscrete agglomerates of particles in the bed (a process termedgranulation).

The extent to which a fluidised bed process is successful depends, interalia, on the uniformity and intimacy with which the contacting phases(gas or vapor, liquid (where present), dispersed solid particles) mix.In many industrial fluidised bed processes this mixing may be markedlynonuniform, and, in the absence of remedies large proportions of theentering gas or vapor establish preferred channels or paths through thebed and escape from it almost without mixing with any of the particlesof the bed.

Methods of remedying this situation are:

1. Provide a bed support which offers a high resistance to the gas orvapor flow. This ensures that the gas or vapor entering the base of thebed of particles will do so at a fairly uniform flow rate.

2. Operate the bed at very high gas rates. This achieves firstly aneffect similar to (1) above and imparts a high degree of turbulence tothe particles in the bed with the result that they mix well, with thegas or vapour.

3. Stir the fluidised bed using paddles.

4. Rotate the grid which supports the bed.

5. Vibrate the bed.

6. Introduce the gas or vapor as a series of pulses instead ofintroducing it at a relatively uniform rate of flow.

These latter four devices ((3), (4), (5), and (6) above) are alldesigned to prevent the establishment of preferred gas or vapor pathsthrough the fluidised bed.

The present invention provides an alternative method of preventing theestablishment of undesirable preferred paths by varying the effectiveresistance of the grid to the flow of the gas or vapor in a programmedmanner utilising means which causes the gas or vapor flow rate enteringthe bed from particular regions of the grid to vary while the gross gasor vapor flow rate remains substantially invariant.

The present invention consists in a method of operating a fluidised bedprocess to prevent the establishment of undesirable preferred paths inthe bed for the fluidising fluid wherein the effective resistance of thegrid of the bed to the flow of the fluid is varied in a programmedmanner, the rate of flow of fluid entering the bed from a particularregion of the grid being varied while the gross fluid flow is maintainedsubstantially invariant.

The present invention further consists in the provision in a fluidisedbed of means for cyclically obstructing the fluid flow through thepoints or passages in the grid, the said means acting to shut off whollyor partly the flow of fluid through regions of the grid in turn.

In order that the invention may be better understood and put intopractice a preferred form thereofis hereinafter described, by way ofexample, with reference to the accompanying drawings in which:

FIG. 1 is a sectional elevation of apparatus incorporating the presentinvention,

FIG. 2 is a cross-sectional view on line II-II of FIG. 1, and,

FIG. 3 is a sectional elevation of apparatus incorporating analternative embodiment of the present invention.

In the apparatus shown in FIG. 1.

10 is a cylindrical vessel containing the fluidised bed, 11 is a thinporous plate on which the bed of particles sit. Its purpose is toprevent particles entering the perforations in the plate, 12 immediatelybelow it. 13 is a rotating louver or flat vane which impedes orcompletely interupts (depending on how close it is to the flat undersideof the perforated plate 12), the flow of gas to the individualperforations in the plate 12 as it rotates. (Items 11, 12, and 13constitute the grid). The rotating louver 13 is driven by a variablespeed electric drive 15 by way of the shaft 14.

The fluidising gas stream enters at the inlet duct 16 into the plenumchamber 17 below the bed, passes through the ports in the rotatinglouver 13, the perforations and the porosities in the plates 12 and 11,enters the bed of granular particles 18 fluidises these and finallyleaves through the exit duct 19 at the top of the cylindrical vessel 10.In operation the rotational speed of the louvre l3 and the gas flow rateare adjusted to obtain a best fluidised bed performance.

FIG. 3 shows an alternative arrangement in which a vessel 20 containinga fluidised bed is provided'with a reciprocating plate 21, to cyclicallyimpede the gas flow to a row of ports in the bed support plate 22 (Inthis figure bubble capsare-shown above each gas port). The rollers 23hold the reciprocating plate 21 against the underside of stationaryplate 2.2. The rod, 24 is attached to plate 21 at one end while theother end is held against a rotating cam 25, by the spring 26. Gasenters the plenum chamber 27 through port 28. In the figure plate 21 isat one extreme of its travel and in principle gas enters the bed throughonly half of the ports and bubble caps in the row shown: gas flow to theremaining (alternatively positioned) ports is a shut-off by the relativejuxtaposition of the ports in plates 21 and 22. (In practice thisshutoff will be incomplete and some leakage may be desirable). When theplate 21 has travelled (following a half revolution of cam 25) to itsother extreme, those ports which were previously shutoff will be fullyexposed to gas flow and those previously exposed will be shut-off. Inoperation the rotational speed of the cam 25 and the gas flow rate areadjusted to obtain a best fluidised bed performance.

The method of the present invention offers advantages over the knownmethods described under the notes (1) to (6) above in that the net gridresistance need not be high and the energy required to obtain asatisfactory performance should be less with the present invention thanin the methods listed under notes (l) to (6) above. It also offersadvantages in that fluidised beds using this method can be operated atfluid rates below the minimum fluidisation rates for conventionallyfluidised beds.

The benefit to be obtained from the use of a method according to theinvention are illustrated by the experimental results described below.

In this experiment a bed of annular cross-section, 5.75 inches outerand4.00 inches inner-diameter was constructed with tubular Perspex walls.(The annular cross section was chosen for experimental convenience inthat it allows a lamp to be placed within the inner (4 inchesouter-diameter) Perspex tube; relative attenuation of the illuminationthrough the surrounding annular bed allows an assessment of howuniformly (or otherwise) the bed is being fluidised.

The bed support plate contained 12 0.375 inch diameter gas ports whosecenters were equally spaced around a 5 inch diameter circle. Each ofthese gas ports was covered with open weave cloth across their top andcontained a small orifice plate and pressure taps to allow individualport gas flows to be separately measured. A vane below the grid had twolobes (instead of three as shown in FIG. 1) each subtending 90 of arc;in operation these obstructed air flow to half of the ports in thesuper-positioned plate.

In the experiments two powders were fluidised, sand and feldspar; theirproperties are given in Table 1. With the vane removed the sandfluidised well, but in the case of the feldspar relatively stable andsubstantially vertical channels formed through the bed and only thepowder in these channels appeared mobile.

Measurements of the variance in gas flow through the bed support plateorifices showed that with the vane removed, minimum variance wasobtained at an air rate 4.1 and 3.8 times the minimum fluidisation raterespectively for the feldspar and the sand, the minimum variance for thefeldspar being 7.5 times that for the sand.

With the vane in position and rotating at speeds between 35 and 60revolutions per minute these minimum variances occurred at 2.5 and 3.3times the minimum fluidisation rate for the feldspar and sandrespectively and the variances obtained were (feldspar) and 50% (sand)of the variances obtained with the vane removed. A particular feature ofoperating with the rotating vane was that a satisfactorily bubbling bedwas obtained for the sand at gross air rates ofO.65 times the minimumfluidisation rate.

TABLE 1 Powder Properties Powder Sand Feldspar Bulk density (lb/cuv ft.)90 65 Angle of repose (degrees) 45 Minimum fluidisation rate (ft/sec.)'0.21 0.17

Mean particle diameter (micro metres) l7O 38 The method according to theinvention may be carried out by means of a variety of different forms ofapparatus other than those described above and these may have orinclude:

Variations of the examples provided may have or include a. ports in therotating or reciprocating louvers of other shapes and a different numberof ports.

b. perforations through the louver vanes.

c. adjacent surfaces (between items 12 and l3 in FIG. 1) and 22 and 23in FIG. 3 which are other than flat.

d. devices in which the grid plates are not circular and in which thelouvers providing the variation in resistance to the gas flow are eitheroscillated or reciprocated.

e. devices in which the fluidising gas is supplied to in dividual gasentry ports at the base of the bed or groups of these ports by way of aseparate, programmed valve system.

f. fluidised bed systems in which the fluidising gas enters the base ofthe bed through ports other than those which may or may not be providedin the bed supporting plates and in which the gas supply is made tofluctuate in the manner described under note (e) above.

g. devices employing more than one rotating, oscillating orreciprocating louver.

h. devices where the louver is guided and moves between two perforatedplates.

I claim:

l. A method of operating a fluidized bed comprising:

varying the effective resistance of the grid of the bed to the flowoffluid in a programmed manner, by selectively opening a portion of someholes while simultaneously closing an equivalent portion of other holeselsewhere in the grid, and continuously changing the holes being openedand closed, whereby gross fluid flow is maintained substantiallyinvariant and preferred channels are avoided.

2. In a fluidized bed apparatus comprising a housing, inlet means,outlet means and a bed support plate having a plurality of holestherethrough, the improvement wherein said apparatus further comprises:

means for cyclically obstructing fluid flow through a number of saidholes and simultaneously allowing fluid flow through the remainder ofsaid holes, the individual holes in said number changing during thecourse of said cycle, leaving the gross fluid flow through the bedsupport plate substantially invari-.

ant throught said cycle.

3. An apparatus in accordance with claim 2, wherein said meanscomprises:

a movable flat plate disposed immediately adjacent to said bed supportplate, said flat plate having a first plurality of holes which can alignwith a first plurality of holes in said bed support plate and a secondplurality of holes which are out of alignment with a second plurality ofholes in said bed support plate when said first pluralities are inalignment, said second plurality of holes in said flat plate beingalignable with said second plurality of holes in said bed support plateupon motion of said flat plate at which time said first pluralities ofholes will be out of alignment; and

means for oscillating said flat plate.

4. An apparatus in accordance with claim 2, wherein said meanscomprises:

a non-perforated flat vane, rotatable around the vertical axis of saidhousing, disposed immediately adjacent to said bed support plate, saidvane, in each position of the rotation thereof, obstructing fluid flowthrough the same proportion of holes in said bed support plate andallowing fluid flow through the remainder of said holes.

1. A METHOD OF OPERATING A FLUIDIZED BED COMPRISING: VARYING THEEFFECTIVE RESISTANCE OF THE GRID OF THE BED TO THE FLOW OF FLUID IN APROGRAMMED MANNER, BY SELECTIVELY OPENING A PORTION OF SOME HOLES WHILESIMULTANEOUSLY CLOSING AN EQUIVALENT PORTION OF OTHER HOLES ELSEWHERE INTHE GRID, AND CONTINUOUSLY CHANGING THE HOLES BEING OPENED AND CLOSED,WHEREBY GROSS FLUID FLOW IS MAIN-
 2. In a fluidized bed apparatuscomprising a housing, inlet means, outlet means and a bed support platehaving a plurality of holes therethrough, the improvement wherein saidapparatus further comprises: means for cyclically obstructing fluid flowthrough a number of said holes and simultaneously allowing fluid flowthrough the remainder of said holes, the individual holes in said numberchanging during the course of said cycle, leaving the gross fluid flowthrough the bed support plate substantially invariant throught saidcycle.
 3. An apparatus in accordance with claim 2, wherein said meanscomprises: a movable flat plate disposed immediately adjacent to saidbed support plate, said flat plate having a first plurality of holeswhich can align with a first plurality of holes in said bed supportplate and a second plurality of holes which are out of alignment with asecond plurality of holes in said bed support plate when said firstpluralities are in alignment, said second plurality of holes in saidflat plate being alignable with said second plurality of holes in saidbed support plate upon motion of said flat plate at which time saidfirst pluralities of holes will be out of alignment; and means foroscillating said flat plate.
 4. An apparatus in accordance with claim 2,wherein said means comprises: a non-perforated flat vane, rotatablearound the vertical axis of said housing, disposed immediately adjacentto said bed support plate, said vane, in each position of the rotationthereof, obstructing fluid flow through the same proportion of holes insaid bed support plate and allowing fluid flow through the remainder ofsaid holes.